The invention generally relates to the field of multi-channel communication devices which are operable at a predetermined plurality of discrete channel frequencies. More particularly, the invention relates to multi-channel communication devices which have step scanning electronic channel selection that either manually or automatically selects one of a predetermined plurality of discrete channel frequencies at which the communication device is operable at.
The most common way to step tune a multi-channel communication device to a plurality of discrete channel frequencies is to use a multi-position rotary mechanical switch which has a different mechanical position for each channel to be selected. This type of prior art mechanical switch is generally used in conjunction with a plurality of different crystals, channel elements, which are selectively actuated according to the position of the multi-position mechanical switch. Multi-position mechanical switches are commonly used as the tuning mechanisms for television sets and citizens band (CB) transceivers.
Some prior art communication devices comprise receivers which are continuously manually or automatically swept tuned across an entire frequency band, the frequency sweeping being terminated when the receiver comes across a desired frequency having a transmitted signal thereon. This type of automatic continuous tuning action, by itself, is totally unsuitable for tuning the receiver to a predetermined channel since if a predetermined channel does not have a signal transmission on it this prior art circuit cannot tune the receiver to the channel. In addition, such continuous tuning is not practical or desireable when the communication device includes a transmitter which must operate at only a predetermined plurality of highly stable discrete channel frequencies.
Automobile car telephone systems have radio equipment which does continually scan a plurality of discrete channel frequencies by sequentially tuning a receiver to each of these frequencies. However, these scanning systems only scan in one direction and are generally fully automatic and do not lend themselves to the use of a manual actuator. In addition, the electronics of the mobile car telephone system is basically concerned with finding open channels and not detecting the presence of a priority channel transmission and producing audible signals in response thereto. Thus standard car telephone equipment cannot tune a radio to a desired predetermined channel.
When a mechanical multi-position switch is used to select one of a predetermined plurality of stable frequency discrete channels, the channel to which the communication device is tuned when power is reapplied to the communication device is the channel that was last previously selected by the mechanical multi-position switch. To have the communication device automatically tuned to a single calling channel whenever power is reapplied to the communication device would be impractical with the prior art switches. Mechanical multi-position switches are generally not reset to one specific channel upon the reapplication of power to a communication device and this function could not be implemented without the use of expensive mechanical drive mechanisms to rotate the multi-position mechanical switch. Electronic tuning systems have generally retained the channel memory function of the multi-position mechanical switch, and no prior art system has enabled the operator of the communication device to select either a previous channel memory function or a calling channel reset function for initially tuning his communication device upon the reapplication of power to the device.
Automatic scanning tuning systems have been provided for communication devices. However, these automatic tuning systems generally latch on to one of a predetermined plurality of channels and further scanning cannot be obtained until the termination of the automatic scanning mode or the termination of signals on the locked in channel. Prior art systems generally do not provide any way for the operator to maintain automatic scanning while manually overriding the automatic scanning when signals on an undesired one of the plurality of channels are being received.
Prior art communication devices which are manually tunable and include transmitter apparatus generally have no apparatus which positively prevents the selection of a different transmitter channel frequency during the transmission of signals by the transmitter. Thus in prior art devices the transmitter channel frequency can be changed during transmission by use of the manual tuning control. Such operation is always undesireable and results in either transmitting on an unintentionally selected channel, creating large transient voltages in the transmitter or possibly creating the transmission of transient frequencies which lie outside of the allowed frequency range for any of the operable channels at which the communication device can operate. Prior art devices provide no way to prevent manually changing the transmitter frequency during the transmission of signals by the transmitter.
Switch bounce eliminator circuits are commonly available and are generally used to provide a response to the movement of a switch actuator which closes a contact. They provided for ignoring and ignores any slight discontinuity in the closure of the contact which may occur directly after the movement of the actuator. This slight discontinuity is referred to as switch bounce. Typically, switch bounce circuits comprise a delay circuit and a Schmidt trigger circuit connected in combination such that the control signal produced in response to the actuator movement is substantially delayed from the time that the switch contact is initially closed. Thus prior art bounce eliminator circuits have not rapidly responded to a switch closure and ignored subsequent discontinuities, but have basically ignored the initial switch closure and the subsequent discontinuity and then produced a control voltage only in response to a subsequent continuous switch closure. These prior art bounce eliminator circuits generally comprise a plurality of logic gates and are relatively expensive. In addition, they do not rapidly respond to the initial movement of the switch actuator. When switches are used to control the manual tuning of a multi-channel communication device, it is desireable that a tuning control signal be rapidly created in response to the movement of a manual tuning actuator, while minimizing the effect of switch bounce. Prior art tunable communication devices have generally totally ignored the effect of switch bounce on tuning switches and thus the tuning of these prior art communication devices is subject to initial variability and slow response because of the existence of switch bounce transients.